With $100, I think I’d buy something from Juniper books. Juniper Books republishes new and old books into themed sets. It was the most excited book related website I found in a while. My favorite sets are the word cloud sets and the penguin classics sets.
I'm an avid reader and love most all books, but my favorite genres tend to be mystery, adventure, science fiction and classics. Genres I tend to shy away from include romance, contemporary fiction and chick lit. If I’m not actually reading books, I’m probably at work shelving them at my local library.